Joachim Wuttke authored Dec 18, 2018 and Joachim Wuttke committed Dec 20, 2018

For add_compile_options and target_compile_options:
- Insert sections.
- Provide cross-links between the two doc pages
- Alleviate and explain links to the commands
  ...compile_definitions and ...include_directories.
- Alleviate link to COMPILE_OPTIONS, and integrate it into
  the synopsis.